This invention relates to clamping devices for use with machinery of the type used to sift, screen, size or separate particulate material, which machines are generically referred to herein as "screening machines".
Screening machines commonly have one or more screens which are mounted in sloping planes within a screen frame or screen box, and the material to be separated is discharged onto the screens. Vibratory motion, reciprocating motion, gyratory motion or combinations thereof are imparted to each screen to shake the material and permit the finer particles to fall through the screen openings while the courser material remains on top of the screen. Such machines are commonly provided with a top cover which encloses the screen assembly. This top cover is releasably clamped to the screen frame or screen box.
The cover encloses the material being screened, preventing it from being shaken off the screen and minimizes dust generated in the screening process which may escape from the screen box. From time to time it is necessary to remove the cover, for instance to change or replace the screen. Because the frame and screen are shaken with substantial force in operation, the cover is typically clamped to the frame for movement with it. The particulate matter to be screened is fed through an inlet chute in the cover which discharges it onto the screen.
Various forms of cover hold-down clamps have been proposed specifically for use on screening machines, including manually operated over-center hold-down clamps, for example the type shown in Nolte, U.S. Pat. No. 3,433,357. In the use of such clamps, a clamp arm is manually engaged with the cover or frame or other member to be clamped and an arm is pulled from one side of a center position to the other side, so as to draw together the two members to be clamped. The standard mechanical clamp is an over-center cam that contains a rigid, threaded rod. This clamp can provide a high clamping force and is a relatively simple design. Such manual clamps provide a strong, non-compensating clamping force.
To provide an approximately uniform clamping force among the several clamps around the periphery of the frame (so that the cover is not held too tightly in one area and too loosely at another), each clamp must be manually adjusted by rotation of the threaded rod to provide roughly the same mechanical clamping force. In a large screening machine there may be as many as 20 or more clamps around the frame. When several such clamps must all be set and adjusted uniformly, it often happens that the force of the earlier set clamps is changed by the later set clamps so that it is necessary to go back and readjust the earlier set clamps. Such individual and repetitive adjustment requires substantial time and effort. Moreover, the threaded rods of such clamps over time become clogged with dust from the material being screened and tend to gall or seize so that they cannot be easily turned and adjusted.
Additionally, when setting mechanical clamps on a screening machine, it is difficult for an operator to set the appropriate clamping force for each of the clamps. Commonly, the operator adjusts the clamp so that it provides the maximum possible clamping force which may result in damage to the equipment. Additionally, to release the clamp when set in this manner, it is often very difficult and may require a pry bar or other mechanical assist and, when the clamp is open, the released force may prove harmful to the operator.
In some applications, screening machines are subjected to substantial heat during use, as for example when a hot material is being screened. Thermal expansion of the frame and/or cover caused by such heat puts a substantial load on the mechanical clamp making it difficult to operate. In some circumstances, the force of thermal expansion on a tightly sealed clamp can even warp or deform the machine components. However, due to the stiff rod, this clamp design provides poor maintenance of clamping force due to dimensional changes associated with temperature, seal setting or configuration between the screen box and the cover and tolerance stack-up such as wear and tear on the equipment.
Another type of clamp which has been used with screening machines is disclosed in Pierson, U.S. Pat. No. 5,150,796 assigned to the assignee of this invention. That patent discloses a clamp which is air pressure operated, both when applying clamping force and to retract or swing the clamp away from a clamping position when it is open. Super-atmospheric pressure is applied to the clamp for clamping and sub-atmospheric pressure is applied to swing the clamp away. This clamp utilizes a pneumatic air stroke actuator that is inflated to hold down the cover.
Advantages of this type of clamp include the avoidance of over clamping which is common with the standard mechanical over-center type of clamp. Additionally, the pneumatic clamp is very compensating to dimensional changes resulting from temperature or other variations while providing consistent clamping force when inflated. Additionally, the pneumatic clamp requires an external air supply unlike standard mechanical clamps. Moreover, the clamping force provided by a pneumatic clamp is more limited than that available from a mechanical clamp. Furthermore, the pneumatic clamps cannot optimally be used in certain environments or with particularly corrosive, or other caustic materials.